Storage-type electroluminescent image amplifier



Aug-31,1965 J. MURR, JR., m1,. 3,204,106

STORAGE-TYPE ELECTROLUMINESCENT IMAGE AMPLIFIER Filed Dec. 28, 1960#Tram/EY United States Patent O 3,204,106 STORAGE-TYPEELECTRULUMINESCENT lMAGE AMPLIFIER .lohn Murr, Jr., Allentown, andHarvey 0. Hook, Princeton, NJ., assignors to Radio Corporation ofAmerica, a corporation of Delaware Filed Dec. 2S, 1960, Ser. No. 7 8,8927 Ciaiins. (Cl. Z50-213) This invention relates to electroluminescentdevices. In particular, this invention relates to a storage lightampliiier type of electroluminescent devices, wherein an 1nput image isvisibly reproduced, in an intensified or arr1- pliiied manner, and inwhich the input image is stored for any selected period of time.

There are several known types of storage light ampl.- fiers whichgenerally include a layer of electroluminescent material and a layer ofphotoconductive material in optical feedback relationship. When apotential is applied across .the series combination of theelectroluminescent and photoconductive materials, most of the potentialdrop occurs across the photoconductor in the absence of an input signal.When the photoconductor is excited by an input image, the radiationreceiving portions of the photoconductor are in a low resistance stateand the potential drop occurs across the electroluminescent layer sothat the electroluminescent material produces `an amplified outpu-timage. Usually, part of the output light from the electroluminescentlayer is fed back to the photoconductor to store the image.

In devices of the type briefly described above, either theelectroluminescent material or the photoconductive material must bedivided into elemental units to prevent spreading of the image. Thereason for this is that light feedback from Ithe electroluminescentlayer is not completely perpendicular to the `surface of theelectroluminescent layer. Thus, adjacent areas of photoconductor areexcited by the feedback light, and, in turn, 'adjacent areas of theelectroluminescent material produce light. If spreading Occurs, and theimage is stored for a long period of time, the image is completelyreplaced by an area of light.

The previous storage light amplifiers have required tedious operation,such as machining, to produce cell barriers to prevent cross-feeding oflight from one elemental unit to the other which triggers on theadjacent cells.

It is therefore an object of this invention to provide an improvedstorage light amplifier.

It is a further object of this invention to provide a novel storagelight amplifier characterized by its simplicity of construction and itsadaptability for economical production.

These and other objects are accomplished in accordance with thisinvention by providing a storage light amplifier wherein a firstelectroluminescent means Iis provided for producing the light output of`the device and a second electroluminescent means is provided forproviding the feedback light to the photoconductive means to providestorage of the image. The second electroluminescent means is opticallyseparated from the first electr luminescent means, by an opaque layer,so that the output side of the device does not provide any lightfeedback and is not directly sensitive to input light. The second orfeedback electroluminescent means is separated into elemental unitsisolated by the photoconductive means so that spreading of the imagedoes not occur.

The invention will be more clearly understood by reference to theaccompanying single sheet of drawings, wherein:

FIG. l is an enlarged fragmentary sectional view of a preferredembodiment of this invention; and,

ICC

FIG. 2 is an enlarged fragmentary sectional View of another embodimentof this invention Referring now to FIG. l, there is shown anelectroluminescent type 1light amplifier 10 comprising a transparentsupport member 12. The transparent support member 12 is shown as being arectangular fiat member but may be of any desired shape orconfiguration. The

transparent support member 12 may be made of any light transparentmaterial such as glass or plastic. Positioned on one surface of thetransparent support member 12 is a light transparent electricallyconducting layer or coating 14. The layer 14 may be any conventionaltransparent conductor such as vapor deposited tin oxide, tin chloride orthin layers of evaporated gold.

Positioned on the transparent conductive layer 14 is a layer 16 of amixture of electroluminescent phosphor yand a suitable binder. Theelectroluminescent phosphor 16 may be any of the knownelectroluminescent materials such as manganese activated zinc sulfideand may be provided in the powdered form when mixed with a suitablebinder such as nitro-cellulose. The electroluminescent layer 16 is forthe purpose of providing an output image from the device 10. Theelectroluminescent layer 16 may, for example, be a continuous coatingapproximately 0.0015 inch thick. The electroluminescent layer 16 may beapplied by spraying, blading or other known techniques. Positioned onthe electroluminescent layer 16 is a light opaque layer 18 which may beapproximately 0.0005 inch thick and which may be made of a material suchas black paint or carbon black in an epoxy resin binder. The lightopaque material 18 may be applied by spraying. The light opaque layer 18is non-conducting in a lateral tliizrection, i.e. parallel to the planeof the support member Positioned on the light opaque layer 13 is adiscontinuous layer comprising a large plurality or multiplicity ifseparate particles of electroluminescent phos-` phor 20 The particles 20may also be made of any suitable known electroluminescent phosphormaterial, one example of which is manganese activated zinc sulfide.VSince the electroluminescent particles 20 are spaced apart, the averagethickness of the array of particles is less than the thickness of oneparticle. Thus, isolated electroluminescent particles are provided. Theisolated electroluminescent particles 20 may be provided by spraying avery dilute electroluminescent paint or dusting a very thin,discontinuous layer of powdered elecrtoluminescent material on a surfacemade tacky by a partly dry lacquer film. When the electroluminescentparticles 20 are applied by spraying, a coating of manganese activatedzinc sulfide in a suitable binder, that is just barely visible to thenaked eye, has been found to be suitable.

Positioned over all of the separate electroluminescent particles 20 is aphotoconductor 22. The photoconduc-` tor 22 is grooved so that a thickphotoconductor may be used for its electrical properties and so thatlight may penetrate to the depths of the photoconductor to excite thephotoconductor, as is known. The photoconductor may be approximately0.008 inch thick, for example, and the grooves may be cut to about 0.004inch from the top of the electroluminescent particles 20. Thephotoconductor may be made of any known photoconductive material, oneexample of which is powdered, copper activated, cadmium sulfide.

Due to the thickness of the photoconductor 22, the light produced by oneelectroluminescent particle 20 is not suicient to excite adjacent areasof the photoconductor to an extent sufficient to cause image spreading.In other words, the body of the photoconductor Z2 effectively functionsas a light opaque shield around each of the isolated electroluminescentparticles Z0.

Positioned on the alternate ridges is a different one of a group ofelectrodes 24. Positioned on the intermediate ridges is a different oneof a group of electrodes 26. All of the electrodes 24 are connected to aterminal 38 while all of the electrodes 26 are connected to a terminal36.

During operation of the device 10, an alternating cur-` rent source, notshown, is connected between the terminal 34 on the transparent conductor14 and a center tap of a direct current source (not shown). The oppositesides of the direct current source are connected through a reversibleswitch to terminals .'56 and 3S. By means of the reversible switch, andthe direct current source, a potential of opposite polarity rnay besequentially applied to all the electrodes 26 and all electrodes 24. Thedirect current source is for the purpose of providing a bias voltage tothe photoconductor 22 so that current -flow is only in one direction.`By reversing the polarity of the direct current source, the storedimage is quickly erased, for example, in 50 milliseconds or less. Thedirect current source may be of a magnitude of approximately 300 voltsD.C. The alternating current source is of sufficient magnitude to causethe electroluminescent layer 16 and the electroluminescent dots 20 toproduce light, when the resistance of the adjacent photoconductor isbelow a predetermined level. The amount of light produced 4by theelectroluminescent dots 20 must be high enough to maintain thephotoconductor 22 in its low resistance state for image storage. Asource of approximately 180 volts at approximately 1,000 cycles persecond has been found to be suicient.

During operation of the device 10, an image to be reproduced is directedinto the photoconductor 22. The image may be a visible image, X-rays orother invisible radiations, with the photoconductor being selected forits high sensitivity to the desired wavelengths.

In the areas of the photoconductor which are struck by the input image,the resistance of the photoconductor is decreased so that theelectroluminescent particles 20 are excited, and the electroluminescentlayer 16 is also excited, since a suicient portion of the potential fromthe alternating current source is now applied across theelectroluminescent layer and the electroluminescent particles 20.

The light produced by the electroluminescent particles 20 is ofsufficient magnitude, or brightness, to feed back to the photoconductor22, in the elemental areas thereof, so that the photoconductor in theilluminated elemental areas is continuously energized and remains at alow resistance state. As long as the input image is stored, the outputelectroluminescent layer 16 will produce a visible amplified outputimage corresponding to the original input image.

Due to the presence of the opaque layerv18, light from theelectroluminescent layer 16 is prevented from feeding back to, andtherefore exciting, the photoconductor 22. Due to the fact that theelectroluminescent particles 20 are spaced apart and isolated from eachother, the light from these particles does not excite adjacentphotoconductive areas so that the image is not diffused or spread. Thereason for this is that the body of the photoconductor 22 functions as alight shield between adjacent electroluminescent dots 20 and imagespreading7 is eliminated. If the feedback electroluminescent means werecontinuous, adjacent areas of photoconductor would be excited, which inturn would excite adjacent areas of electroluminescent areas so that theimage would spread and eventually become a continuous light source.

In addition to the conventional storage of an image previouslydescribed, other interesting effects have been noted in panels of thetype shown in FIG. l. For example, after the photoconductor has beenexposed to an image, by reversing the polarity of the electric field onthe interdigited electrodes 24 and 26, and at the same time by exposingthe input photoconductor to a uniform light source (not shown), thenegative of the original stored image is produced on the outputelectroluminescent phosphor 16. The reason for this is that thephotoconductor is less sensitive, in the previously exposed areas thanit is in the previously un-exposed areas. This negative image may alsobe stored indefinitely once it has been established by the describedprocedure.

Another interesting effect is that, by over exposure of the panel to theuniform light source only the border or outline of the original image isretained. It is believed that this effect occurs because of a fatigueeffect in the center exposed portions of the photoconductor, when thesensitivity of the photoconductor is reduced below the level at whichstorage is possible. By means of this phenomena, only the outline of theoriginal stored image is produced on the electroluminescent layer 16.This outline may also be stored indefinitely.

Halftone pictures may be displayed on the storage device 10 by operatingat an alternating current voltage level that is slightly below the levelat which full image storage is obtained. Under these conditions, thepicture fades to black, although the halftone picture takes severalminutes until it decays to where it is no longer visible.

Referring now to FIG. 2, there is shown another embodiment of thisinvention. This embodiment differs from that shown in FIG. l only inthat the feedback electroluminescent means is divided into predeterminedspaced apart areas. The predetermined spaced apart areas ofelectroluminescent phosphor 40 may be provided on the opaque layer 18 byholding a metal mesh screen (not shown) in Contact with the opaque layer13, while the electroluminescent areas 40 are sprayed through the mesh.Then, when the mesh screen is removed, an array of separate, isolatedelectroluminescent areas 40 is provided. The operation and materials ofthe embodiment shown in FIG. 2 may be similar to that described inconnection with FIG. l.

What is claimed is:

1. An image amplifier comprising a conductive layer, anelectroluminescent layer, an opaque layer, a discontinuouselectroluminescent layer, and a continuous photoconductive layer, saidlayers being substantially coextensive and being in electrical contactwith each other in the order named, and at least one electrode on saidphotoconductive layer.

2. An image amplifier as in claim 1 wherein said discontinuouselectroluminescent layer is in light exchange relationship with saidphotoconductor and comprises a plurality of separately isolatedelemental areas of electroluminescent phosphor.

3. An image amplier as in claim 1, wherein said discontinuouselectroluminescent layer comprises a discontinuous coating ofspaced-apart electroluminescent particles.

4. An electroluminescent light amplifier comprising a transparentsupport member, a light transparent electrical conductor on said supportmember, a continuous layer of electroluminescent phosphor on saidtransparent conductor for providing light output from said lightamplier, a continuous layer of light opaque material on saidelectroluminescent phosphor, a plurality of electroluminescent particlesspaced apart on said layer of light opaque material, and aphotoconductor isolating each of said plurality of electroluminescentparticles from any other.

5. An electroluminescent light amplifier as in claim 4 wherein saidphotoconductor includes a plurality of grooves extending substantiallyparallel to the plane of said support member, and means for connectingthe opposite polarities of a direct current source to saidphotoconductor.

6. An electroluminescent light amplifier comprising a light transparentsupport member, a light transparent electrically conductive coating onsaid support member, a continuous layer of electroluminescent phosphormaterial on said transparent conductive coating, a continuous layer oflight opaque material on said layer of electroluminescent phosphor, adiscontinuous layer of spacedapart electroluminescent particles on saidlayer of light opaque material, and a photoconductor on said array ontelectroluminescent particles and on the light opaque layer therebetween.

7. An electroluminescent light amplifier as in claim 6 wherein saidphotoconductor includes a plurality 0f grooves, the alternate ridges ofsaid photoconductor having a first set of conductors thereon, theintermediate ridges of said photoconductor having a second set ofconductors thereon, said sets being adapted to have the alternatepolarities of a direct current source applied thereto.

References Cited by the Examiner UNITED STATES PATENTS 2,914,678 ll/59Kazan Z50-213 2,928,006 3/60 Kruse 250-213 2,957,991 10/60 Kazan 250-2132,997,596 8/61 Vize 250-213 X 2,999,165 9/61 Lieb 250-213 2,999,941 9/61Klasen et al. Z50-213 3,002,102 9/61 Palmer Z50-213 FREDERICK M.STRADER, Primary Examiner.

ARCHIE R. BORCHELT, RICHARD M. WOOD,

RALPH G. NILSON, Examiners,

1. AN IMAGE AMPLIFIER COMPRISING A CONDUCTIVE LAYER ANELECTROLUMINESCENT LAYER, AN OPAQUE LAYER, A DISCONTINUOUSELECTROLUMINESCENT LAYER, AND A CONTINUOUS PHOTOCONDUCTIVE LAYER, SAIDLAYERS BEING SUBSTANTIALLY COEXTENSIVE AND BEING IN ELECTRICAL CONTACTWITH EACH OTHER IN THE ORDER NAMED, AND AT LEAST ONE ELECTRODE ON SAIDPHOTOCONDUCTIVE LAYER.